Hearth roll and manufacturing method therefor
A hearth roll includes a base roll, a thermally sprayed coating formed on the base roll, and a modified coating formed on the thermally sprayed coating. The modified coating is formed by modifying a part or the whole of a surface of the thermally sprayed coating by melting and solidification of the thermally sprayed coating, by irradiating a part or the whole of the surface of the thermally sprayed coating with an energy beam. The thickness of the modified coating is from 2 to 20 μm, and the Vickers hardness HV of the modified coating is from 1.2 to 1.4 times larger than the Vickers hardness HV of the thermally sprayed coating.
1. A hearth roll, comprising:
a base roll;
a thermally sprayed coating formed on the base roll; and
a modified coating formed on the thermally sprayed coating, the modified coating being formed by modifying a part or the whole of a surface of the thermally sprayed coating by melting and solidification of the thermally sprayed coating, by irradiating a part or the whole of the surface of the thermally sprayed coating with an energy beam,
a thickness of the modified coating being from 2 to 20 μm, and
a Vickers hardness HV of the modified coating being from 1.2 to 1.4 times larger than a Vickers hardness HV of the thermally sprayed coating.
2. The hearth roll according to claim 1 , wherein cracks are present on a surface of the modified coating, and an average spacing between adjacent cracks in a cross-section of the hearth roll cut in a thickness direction is from 10 to 100 μm, and opening widths of the cracks are less than 5 μm.
3. The hearth roll according to claim 1 , wherein the modified coating comprises from 0.5% to 2% by mass of oxygen.
4. The hearth roll according to claim 1 , wherein Al 2 O 3 is present in a dispersed state in a surface of the modified coating, and a proportion of an area of Al 2 O 3 in the surface of the modified coating is from 5% to 40%.
5. The hearth roll according to claim 1 , further comprising a chromium oxide layer formed on the modified coating, or on the modified coating and the thermally sprayed coating.
6. The hearth roll according to claim 1 , wherein the thermally sprayed coating is a cermet coating consisting of a heat-resistant alloy and a ceramic,
the ceramic including, in terms of % by volume, Cr 3 C 2 at from 50% to 90%, Al 2 O 3 at from 1% to 40%, Y 2 O 3 at from 0% to 3%, and ZrB 2 at from 0% to 40%, and the balance being composed of impurities and pores,
the heat-resistant alloy including, in terms of % by mass, Cr at from 5% to 20%, Al at from 5% to 20%, and at least one of Y or Si at from 0.1% to 6%, and the balance being composed of at least one of Co or Ni and impurities, and
from 50 to 90% by volume of the cermet coating being the ceramic, and the balance being the heat-resistant alloy.
7. The hearth roll according to claim 6 , wherein the heat-resistant alloy further includes, in terms of % by mass, at least one of Nb at from 0.1 to 10% or Ti at from 0.1 to 10%.
8. A method of producing the hearth roll according to claim 1 , comprising a step of irradiating a part or the whole of a surface of a thermally sprayed coating formed on the base roll with an energy beam, thereby modifying a part or the whole of the surface of the thermally sprayed coating by melting and solidification of the thermally sprayed coating, to form a modified coating having a thickness of from 2 to 20 μm and a Vickers hardness HV that is from 1.2 to 1.4 times larger than the Vickers hardness HV of the thermally sprayed coating.
9. The method of producing a hearth roll according to claim 8 , wherein the irradiating with the energy beam is performed in the atmosphere.
10. The method of producing a hearth roll according to claim 8 , wherein a chromate treatment is performed after the modified coating is formed.